1. Field of the Invention
The present invention relates to a ternary signal input circuit for converting an input ternary signal into binary signals and outputting the binary signals.
2. Description of the Related Art
Communication systems which employ a ternary signal such as an AMI (Aletrnate Mark Inversion) code that is used by ISDN (Integrated Services Digital Network) basic interfaces require receiver circuits capable of receiving three potentials of input voltages.
Heretofore, ternary signal input circuits for use as receiver circuits for communications employing a ternary signal comprise a circuit composed of analog devices.
As shown in FIG. 1 of the accompanying drawings, such a conventional ternary signal input circuit comprises a transformer 10 for being supplied with a signal from a communication line, resistors 73, 74, 75 connected in series with each other between a power supply Vdd and ground, an analog comparator 76 having a first input terminal connected to the transformer 10 and a second input terminal connected to a junction between the resistors 73, 74, and an analog comparator 77 having a first input terminal connected to a junction between the resistors 74, 75 and a second input terminal connected to the transformer 10. The analog comparators 76, 77 have respective output terminals serving respectively as output terminals 78, 79 of the ternary signal input circuit.
Operation of the conventional ternary signal input circuit will be described below with reference to FIG. 2 of the accompanying drawings.
A signal from the communication line is supplied through the transformer 10 having a terminal 12 to which a potential equal to one half (1/2) of the voltage of the power supply Vdd.
As shown in FIG. 2, an input signal 11 that has passed through the transformer 10 selectively has the 1/2 Vdd potential, a (+) potential, and a (-) potential. These three alternative potentials represent data carried by the input signal 11.
In order to process the input signal with a digital circuit, it is necessary to convert the input signal into a binary signal having two alternative potentials such as a 0 potential and a Vdd potential.
Consequently, a signal converter for converting a ternary signal of three potentials into a binary signal of two potentials is necessary.
As shown in FIG. 1, the power supply voltage Vdd is divided by the three resistors 73, 74, 75 into a potential 71 slightly higher than the 1/2 Vdd potential and a potential 72 slightly lower than the 1/2 Vdd potential. These potentials 71, 72 are supplied to the analog comparators 76, 77, respectively, which compare the supplied potentials 71, 72 with the input signal 11 thereby to convert the ternary signal into a binary signal.
As shown in FIG. 3 of the accompanying drawings, if the input signal 11 is higher than the potential 71 that is slightly higher than the 1/2 Vdd potential, then the analog comparator 76 outputs the Vdd potential (see FIG. 2), and if the input signal 11 is lower than the potential 72 that is slightly lower than the 1/2 Vdd potential, then the analog comparator 77 outputs the Vdd potential (see FIG. 2). If the input signal is of a potential between the potentials 71, 72, then both the analog comparators 76, 77 output the 0 potential.
In this manner, the conventional ternary signal input circuit shown in FIG. 1 converts a ternary signal into two binary signals.
However, the conventional ternary signal input circuit suffers the following problems:
(1) Since usual digital devices have only two potentials such as the 0 potential and the Vdd potential, they cannot handle an input signal having three potentials. Therefore, the conventional ternary signal input circuit, which comprises analog devices, is complex in circuit arrangement, is made up of many components, and is expensive to manufacture.
(2) Analog devices such as analog comparators need many transistors as internal components. Because those transistors cannot individually be reduced in size with ease, the area taken up by the circuits on an LSI chip is relatively large. Consequently, when the circuits composed of those analog circuits are incorporated into an LSI chip, the LSI chip is relatively large in size and expensive to manufacture.